Mikael Wrang
Test Services Sales Manager, Interfleet Technology AB
THE INTERACTION between rail and vehicle is fundamental to the rail mode. Without an understanding of the dynamic forces acting in the contact patch between wheel and rail, it would have been difficult to achieve the increases in speed, cant deficiency and axleload seen during the last 50 years.
Accurate measurement of dynamic forces between wheel and rail has been crucial, and Instrumented Wheelset Technology (IWT) has been in use since the 1950s. The development can be traced back to a UIC competition seeking solutions to the problem of hunting, when Swedish State Railways proposed the use of instrumented wheelsets to measure contact forces.
This first wheelset could only measure lateral wheel-rail forces, but in 1961-62 a wheelset that could measure both lateral and vertical forces was introduced, also in Sweden. Over the following 40 years, IWT has gradually developed into a reliable and widely-accepted tool for investigating wheel-rail forces.
Today, the technique is used for:
- approval of a vehicle's running safety against derailment and assessment of the track damage caused by the vehicle;
- demonstrating safety and financial benefits when enhancing capacity in terms of speed or axleload;
- developing solutions to phenomena caused by wheel-rail forces, such as rolling contact fatigue and crack propagation in rail and wheel;
- infrastructure mapping - IWT developments in recent years have allowed extremely high frequency (up to 2kHz) force inputs to be recorded, such as those generated by rail corrugations;
- derivation of design load spectra for railway vehicles and track structures;
- validation of computer models and tools used to simulate vehicle dynamics.
Expanding the market
When Interfleet Technology acquired the laboratory and testing facilities of the former Swedish State Railways in 2003, which had many years of experience with instrumented wheelsets, the company looked at the potential for expanding the market.
Analysis revealed that the market for IWT would be much larger if the technology could be made more competitive. Historically, three issues have limited the use of instrumented wheelsets.
Firstly, there is a certain cost for obtaining specially-designed wheels with curved webs, and for their subsequent instrumentation, as an extremely high degree of accuracy is required when bonding strain gauges to the webs. Secondly, there is often a long lead time, mainly due to the procurement of the special wheelsets - a period of up to 30 weeks is not uncommon.
An instrumented wheelset cannot be used in revenue service after testing has been completed, because the wheels are non-standard and the instrumentation requires holes to be drilled in the axle to allow strain gauge cables to be run to both sides of the web. Despite the proven safety of such wheelsets, their non-standard features can require additional safety clearance on some railways.
Starting from its third-generation IWT, Interfleet has developed IWT4 which allows virtually any standard production wheelset to be instrumented, avoiding both the cost and delay of designing or manufacturing special wheels. A prerequisite for this is advanced post-assembly calibration procedures, which also offer a reduction in the accuracy required for locating strain gauges on wheel webs, further reducing the cost and time required for assembly.
Using wireless transmission and instrumenting only one side of the wheel avoids the need to drill holes in the axle or the wheels themselves. And because the wheelset is a standard production component which can be returned to service after the tests, the expense of additional safety clearance procedures can be avoided.
The wireless telemetry system has been developed using commercially-available components, but with a bespoke system design. A new 'black box' calibration procedure is used, which does not require any pre-analysis of the wheel such as finite element analysis. It also removes the need for curved wheel webs and allows for greater flexibility as to web shape and when bonding gauges.
IWT4 also offers greater flexibility of use. The lateral contact point position as well as the longitudinal creep forces can be measured, both regarded as critical when assessing rolling contact fatigue.
The one area where IWT4 cannot yet match IWT3 is in the measurement of forces at very high frequencies, although it should be suitable for most applications.
IWT4 has been undergoing validation and proving trials in Sweden and Finland during 2006 and will be exhibited at InnoTrans in Berlin this month. It is planned to go into commercial use before the end of this year.
